1. Field of the Invention
The present invention relates generally to packaging of semiconductor dice and, more specifically, packaging of semiconductor dice to provide improved heat dissipation characteristics.
2. State of the Art
During operation, semiconductor devices typically generate large amounts of heat. The amount of heat that a semiconductor device generates is typically related, if not proportional to, the density of features of the semiconductor device. Heat reduces the reliability with which semiconductor devices, including processors and memory devices, operate. In addition, the exposure of semiconductor devices to elevated temperatures for prolonged periods of time may also decrease the useful lives thereof. Accordingly, the dissipation of heat from semiconductor devices has long been a concern in the semiconductor device industry.
The reduced power requirements of state-of-the-art semiconductor dice have been useful for decreasing the amount of heat generated by such semiconductor dice. Nonetheless, as feature densities are ever-increasing, the temperatures generated by semiconductor dice with even reduced power requirements will also continue to increase. Thus, heat dissipation continues to be of concern, even with the low power requirements of state-of-the-art semiconductor dice.
When a semiconductor die is encapsulated, or packaged, the most delicate regions thereof, such as the active surface that bears integrated circuitry and the bond wires that connect bond pads of the semiconductor die to corresponding leads of a lead frame or contacts of a carrier substrate, are covered with a dielectric protective material. In addition, other, more robust surfaces of the semiconductor die, such as the peripheral edges and back side thereof, are also covered with the dielectric protective material. Unfortunately, many of the dielectric protective materials that are used to encapsulate semiconductor dice are not good heat conductors. As a result of the manner in which such dielectric protective materials have been used to coat semiconductor dice, a large amount of the heat generated by an encapsulated semiconductor die becomes trapped within or around the die.
Several approaches have been taken to improve the rate at which heat is transferred and dissipated from packaged semiconductor devices. Conventionally, large surface area structures formed from materials that have good heat conductivity properties and, thus, which are able to “pull” or transfer heat away from a structure, such as a semiconductor die, contacted thereby have been used to dissipate heat from the package during operation of the semiconductor die or dice thereof. These large surface area structures are generally known in the art as “heat sinks.” Air circulation systems, which often include cooling fans, have also been used, typically in combination with heat sinks or other heat dissipation means. While heat sinks and air circulation systems may be useful for maintaining conventionally configured semiconductor dice at acceptable operational temperatures in some applications, heat sinks are typically fairly massive and the size thereof prevents further increases in the densities at which semiconductor devices are carried upon circuit boards, as is desired to maintain the trend for ever-decreasing electronic device sizes. In addition, heat sinks may also present locational problems between adjacent, superimposed circuit boards and for space-critical applications, such as laptop and notebook computers, cell phones, personal digital assistants and the like.
As an alternative to the use of space-consuming heat sinks, encapsulation processes have been modified to reduce the amount of dielectric protective material that covers the surfaces of semiconductor dice. Additionally, encapsulation techniques have been developed that protect the most delicate portions of a semiconductor die, while leaving other surfaces of the semiconductor die bare, thereby improving heat dissipation therefrom.
One such technique is described in U.S. Pat. No. 5,604,376 to Hamburgen et al. (hereinafter “Hamburgen”), which describes a packaged semiconductor device in which a back side of a semiconductor die is exposed through an encapsulant to facilitate the dissipation and transfer of heat from the back side of the semiconductor die. The packaged semiconductor device of Hamburgen also includes leads to which bond pads of the semiconductor die are electrically connected. The assembly and packaging method described in Hamburgen includes temporarily securing a bare semiconductor die upon a pedestal by application of a vacuum through the pedestal to a back side of the semiconductor die. Leads are then electrically connected to corresponding bond pads of the semiconductor die by way of conventional wire bonding processes. Next, the assembly is positioned over a bottom half of a mold, with the back side of the semiconductor die resting upon a platform. Upon enclosing the semiconductor die and the bond wires within a cavity of the mold and as a molding compound is introduced into the cavity, a negative pressure is applied through an aperture in the platform to the back side of the semiconductor die, causing the back side of the semiconductor die to be pulled against the platform and purportedly preventing the molding compound from flowing onto the back side of the semiconductor die. This process may be somewhat undesirable for several reasons. For example, as the semiconductor die and the mold platform therefor are both rigid structures, any deviations in the planarity or mutual orientation of either the back side of the semiconductor die or the surface of the platform may permit molding compound to flow therebetween. Such planarity deviations, coupled with the force applied to the semiconductor die to temporarily secure the same to the mold platform, may also exert potentially damaging stresses on the semiconductor die during the encapsulation process.
Another example of a packaged semiconductor device that includes a semiconductor die with an exposed back side is described in U.S. Pat. No. 6,348,729 to Li et al. (hereinafter “Li”). The packaged semiconductor device of Li is formed by attaching an adhesive-coated tape or film to a surface of a lead frame and securing a semiconductor die to the adhesive-coated tape or film, within a centrally located opening of the lead frame. Bond pads of the semiconductor die are then electrically connected with corresponding leads of the lead frame by forming or positioning intermediate conductive elements (e.g., bond wires) therebetween. Next, the semiconductor die, intermediate conductive elements, and regions of the leads that are located adjacent to the semiconductor die and above the tape or film are encapsulated. Finally, the tape or film is removed from the packaged semiconductor device structure (e.g., by peeling). Unfortunately, in addition to exposing the back side of the semiconductor die, surfaces of the leads are also somewhat undesirably exposed. Exposure of the bottom surfaces of the leads may increase the likelihood of electrical shorting between leads as the packaged semiconductor device is positioned upon a carrier substrate, such as a circuit board. Moreover, upon securing the packaged semiconductor device of Li to a carrier substrate, the back side of the semiconductor die thereof will be positioned adjacent or very closely to the carrier substrate, which may hinder the dissipation of heat from the back side of the semiconductor die, defeating the intent of exposing the back side.
During the preliminary stages of semiconductor device fabrication processes, the back sides of silicon wafers and other bulk semiconductor substrates are typically adhered to a preformed dielectric protective film, such as a polyimide film. In addition to protecting the back sides of substrates during fabrication processes and as the substrates are being handled and transported from one fabrication process location to another, these dielectric protective films also retain the positions of the various semiconductor devices that have been fabricated on a particular semiconductor substrate following singulation of the semiconductor devices, which are, at this point, commonly referred to as “dice,” from one another. The dice may then be tested or otherwise evaluated, and operable, useful dice picked from the dielectric protective film for further testing, assembly, or packaging.
The inventors are not aware of structures that facilitate heat dissipation from a back side of a semiconductor die through a molded encapsulant while reducing compressional stresses on the semiconductor die during encapsulation thereof and without undesirably increasing the size of the packaged semiconductor device or causing electrically conductive structures from being undesirably exposed through the encapsulant.